Physics A2 unit 5, section 9.3 - more about alpha, beta and gamma radiation

In this section:

• what happens to the nucleus when it undergoes a radioactive change?
• how does the intensity of gamma radiation change as it spreads out?
• how should we represent the change in a nucleus when it emits radiation?

The nature of alpha, beta and gamma radiation

Alpha radiation:

• overall has a positive charge
• made up of two protons and two neutrons
• so it's a helium 2- ion (Rutherford)

Beta radiation:

• two types, only ß- occurs naturally
• ß- natural type: negative charge; emits an electron when an isotope with too many nutrons decays by converting a neutron to a proton
• ß+ second type: positive charge; emits a positron when an isotope with too few neutrons decays by convering a proton to a neutron
• We get the second type as a result of creating new isotopes through high energy proton collisions in particle accelerators

Gamma radiation:

• no charge
• photons with wavelength ~10-11m.

The inverse square law for gamma radiation

The Intensity I of gamma radiation is the radiation energy per second passing through a unit area.

• imagine you have a source of gamma radiation that's small enough to consider to be a single point
• it emits some number of photons per second - let's call that n
• each of these photons has energy hf where h

  Plank's constant 6.626 × 10-34 J•s and f

  the frequency of the radiation
• energy of a photon calculator at PV Education
• so the release of energy per second from the source is nhf
• now imagine a large sphere with a centre at the point source and radius r - it has surface area A = 4(pi)r2
• since the photons released pass through the surface of the sphere 'on the way out', their total energy is shared equally across this area
• so intensity of radiation = radiation energy per second ÷ surface area of sphere at a particular distance r

Equations for radiactive change

Consider the nucleus of a radioactive isotope, the nuclide:

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